Looks like the Ion's tires are about 3% larger.
http://tire-size-conversion.com/tire-size-comparison/
Like Balto said, that would likely help your highway mileage. Another thing to consider is that you'll be giving the Ion's new owner smaller than stock tires. Shouldn't really matter to most buyers, but I would be sure it doesn't hurt the handling and notify the buyer of the different size.